Not Applicable
Not Applicable
1. Field of Invention
This invention relates to a sundial, specifically to a modified equatorial sundial, which gives sidereal time, i.e. star time, as well as solar and watch time.
2. Description of Prior Art
At least two classes of sundials giving sidereal time have been described in the literature. They differ dramatically from each other and from my proposed sidereal sundial. H. Michnik introduced the bifilar type, which uses the intersection of the shadows cast by two crossed threads upon a dial face, in Astronomische Nachrichten, 216, 441 (1922). F. Sawyer has elaborated on Michnik""s work in three articles on xe2x80x9cBifilar Gnomonicsxe2x80x9d published in the Journal of the British Astronomical Association, June 1978, 88(4): 334-351 and in Bulletin of the British Sundial Society, February 1993, 93(1): 36-44, and February 1995, 95(1): 18-27. His sundial requires the use of a separate complex printed map of hour lines for determining sidereal time. A typical beginning astronomy student would not understand why a bifilar sundial functions the way it does or follow the mathematics used to describe its construction.
J. Bores has proposed a sidereal sundial with a conical gnomon that casts a shadow on yet another maze of sidereal hour lines (xe2x80x9cSundials With Conical Gnomons For Sidereal Timexe2x80x9d in Journal of the North American Sundial Society, September 1998, Vol 5, No 3, pp. 25-29). Again, a student would not understand either the basis of operation or the mathematics involved.
And, finally, R. Vinck has briefly described yet another sidereal sundial using a conical gnomon in xe2x80x9cA Sundial For Sidereal Timexe2x80x9d ibid, pp. 29-31; however its theoretical basis and operation are not clearly presented.
The most relevant prior art is a letter by A. Parkin in Sky and Telescope magazine, Apr. 22, 1992, pp. 365 and 366. Parkin shows an ecliptic disk mounted on a rotatable gnomon that is used to illustrate the existence of the ecliptic plane when this disk casts a thinnest shadow. But this is as far as Parkin went; he did not introduce an equatorial disk or seek to determine sidereal time.
Accordingly, several objects and advantages of my sidereal sundial are:
(a) to provide a simple apparatus, using a rod for a gnomon, that is easy to understand and operate for determining sidereal time using the sun;
(b) to require a minimum amount of mathematics to understand its construction, setting, or operation;
(c) to provide a demonstration apparatus suitable for a beginning astronomy course;
(d) to create a new type of sundial based on the classic equatorial sundial.
My sidereal sundial is a modified equatorial sundial, with an opaque ecliptic plane mounted at an angle of 23.4xc2x0 with respect to an equatorial plane. Rotating these planes jointly to minimize the shadow being cast by the ecliptic plane yields the location of the vernal equinox and by extension, the local sidereal time.